1. Field of the Invention
The present invention relates to a DC-DC Converter for supplying a power of a DC power source to a DC load or a DC power source, and a secondary battery charge and discharge system including the same, and a method of controlling the DC-DC Converter.
2. Description of the Related Art
Recently, systems including a DC power source such as a secondary battery, a solar cell, a fuel cell, etc. have been developed due to increase demands for global environment conservation. These systems require a DC-DC converter for supplying a power from a DC power source to a load, or another DC power source.
An abstract of JP2008-79403 A discloses “semiconductor switching devices Q1 to Q4 are disposed between a DC power source and a transformer, and a rectifying circuit and an output smoothing circuit are disposed between the transformer and the load. JP2008-79403 A discloses as follows:
An RCD snubber circuit is disposed between a rectifying circuit and an output smothering circuit to absorb a serge voltage included in an output side voltage V1 of the rectifying circuit through a snubber diode Ds. The charges in the snubber capacitor are discharged through a snubber resistor and supplied to a load as an electric power.
However, because supplying the charges stored in the snubber capacitor for clamping the surge voltage to the load as an electric power is done through the snubber resistor, the snubber resistor consumes the energy.
On the other hand, JP 2009-55747 A discloses a bi-directional DC-DC converter including a transformer for connecting a voltage type full bridge circuit connected to a first power source and a current type switching circuit connected to a second voltage source. Further, a voltage clamp circuit including switching devices and clamping capacitor is connected to the current type switching circuit and there is provided with a control circuit for cooperatively operating the switching devices so as to control a current flowing through a resonation reactor. Non-patent document K. Wang, C. Y. Lin, L. Zhu, D. Qu, F. C. Lee and J. S. Lai, “Bi-directional DC to DC Converters for Fuel Cell Systems”, IEEE power electronics in transportation, IEEE, 1998, pp. 47-51 discloses a configuration similar to JP 2009-55747 A.
These DC-DC converters perform a step-down operation (bucking operation) for supplying power from the voltage type circuit to the current type circuit and a step-up operation (boosting operation) for supplying a power from the current type circuit to the voltage type circuit. These DC-DC converters discharge not through the snubber resistor, but through clamp switching devices in ON states when the charges in the clamp capacitors are discharged, so that an energy loss by the snubber resistor can be avoided previously.
Hereinafter the step-down operation throughout the specification means supplying a power from the voltage type circuit to the current type circuit. The boosting operation means supplying the power from the current type circuit to the voltage type circuit.
In the DC-DC converters, generally, the operation is started from a status in which an output is restricted to prevent an excessive current from flowing through the circuit when the power conversion operation starts and after that the output is gradually increased. In the DC-DC converters disclosed in JP2008-79403A and JP2009-55747A, when the step-up operation (boosting operation) is started in the status in which the output is restricted, a duty ratio of the switching devices in the current type circuit is low and a duty ratio of the clamp switching circuit is higher.
However, in these DC-DC converters, the clamp capacitor is charged in step-down operation at a voltage higher than the output voltage during the operation. Further, the clamp capacitor is charged at a voltage higher than an input voltage during the step-up operation. The clamp capacitor holds the voltage for a predetermined period after the step-down and step-up operations. Accordingly, there may be a case where the step-up operation is started when the step-up operation is started in a state that the clamp capacitor is charged at a high voltage, an excessive current may flow through the clamp switching device or a smoothing inductor because of the high ratio of an ON period of the clamp switching device.
To suppress the excessive current, there is a method of controlling a current in which the charges in the clamp capacitor are gradually discharged through a resistive component and after the voltage drop of the clamp capacitor the step-up operation is started. In this method, there is a problem in that a predetermined period is required up to start of a next step-up operation after the stop of the step-down and step-up operation.
To suppress the excessive current, it may be possible to add a circuit for discharging for the clamp capacitor separately. This will increase a cost.